1. Field of the Invention
The present invention relates to an intake air flow rate measuring device of an internal combustion engine that measures the intake air flow rate of the internal combustion engine that is mounted on an automobile, for example, and is used for controlling the amount of fuel such as gasoline to be mixed into the intake air.
2. Description of the Related Art
Intake air flow rate measuring devices of the above kind are disclosed in JP-A-2002-5710, for example. An intake air flow rate measuring device that is shown in FIG. 5 of this publication is provided with a shunt pipe that projects into an intake passage of an internal combustion engine. The shunt pipe has a first passage extending approximately parallel with the intake passage, a shunt plate projecting into the first passage, a second passage that is formed around the shunt plate and bypasses the first passage, and an air flow rate measuring element disposed in the second passage. The air flow rate measuring element measures the intake air flow rate of the internal combustion engine in the intake passage on the basis of the flow rate or flow speed of the air in the second passage.
FIG. 1 of JP-A-2003-176740 shows an intake air flow rate measuring device in which a dust interrupting means such as a louver is disposed at the inlet of a bypass flow passage where an air flow rate measuring element is disposed.
However, in the intake air flow rate measuring device shown in FIG. 5 of JP-A-2002-5710 in which the shunt plate projects into the first passage, if the air passing through the intake passage contains dust or liquid matter, the dust or liquid matter is stopped by the shunt plate and introduced, together with the intake air flow, into the second passage where the air flow rate measuring element is disposed. The dust or liquid matter introduced into the second passage sticks to the side walls of the second passage or the air flow rate measuring element and disables correct intake air flow rate measurement.
In the intake air flow rate measuring device disclosed in JP-A-2003-176740, the dust interrupting means such as a louver is disposed at the inlet of the bypass flow passage where the air flow rate measuring element is disposed. This publication explains that the dust interrupting means decreases the kinetic energy of dust and thereby prevents a break of the measuring element even if the dust reaches it. However, intake air flow rate measuring devices provided with the dust interrupting means have problems that the pressure loss is large and that an intake air flow is disordered and made unstable by the dust interrupting means, which results in decrease in air flow rate measurement accuracy.